Centrifugal pumps are used for pumping a wide variety of liquids and suspensions. The pumps used for pumping clean liquids differ a great deal from the pumps used for pumping suspensions or even substantially large sized solid particles like fish, for instance. When pumping liquids it is the head and the efficiency ratio that normally count. But when pumping suspensions or solids in liquid, the properties of the solids start playing an important role. The larger the solid particles are the bigger is their role in the design of the pump. In some applications, the solid particles to be pumped should be handled with care, i.e. such that the pumping does not break the particles. In some other applications the purpose may be the opposite. For instance in pumping sewage slurries the pumps are often provided with some kind of breaking means for chopping the solids into smaller particles. And sometimes the fluid to be pumped contains solid particles that tend to block the pump. In such a case the fluid to be pumped contains long filaments, threads, strings or other lengthy flexible objects that easily adhere to the leading edge of the impeller vanes and start collecting other objects so that a thicker rope-like object is formed. Such an object not only grows larger and larger blocking gradually the vane channels, but also easily gets into the gaps between the impeller vanes and the pump housing increasing the power needed to rotate the impeller, and causing mechanical stress to both the shaft of the pump, the coupling between the pump and the drive motor, and the impeller vanes.
A yet further type of fluids pumped by means of a centrifugal pump is fibrous suspensions of pulp and paper industry. In such a case the fibers or particles of the suspension are relatively small, i.e. the length of the fibers being of the order of a fraction of a millimeter to about 10 millimeters. Such fibrous suspensions are not normally able to block the pump, but it has been, however, learned that the fibers tend to adhere to the leading edge of an impeller vane of an ordinary centrifugal pump. Here, an ordinary centrifugal pump is supposed to have vanes of a traditional water pump, in other words vanes, whose leading edges are sharpened, i.e. thinner than the rest of the vane thickness. The problem of fibers adhering to the leading edges of the vanes has been discussed in GB-A-1412488. The problem has been solved by thickening the leading edge of the vane such that the diameter of the thickened leading edge is larger than the thickness of the rest of the vane. This structural feature together with the increased turbulence achieved by a change in the inlet angle of the impeller vane prevents fibers from adhering to the leading edge of the vane.
On the one hand, the above discussed GB-document does not teach the actual problem related to the fibers adhering to the leading edge of the vanes, and, on the other hand, does not even recognize that a similar problem appears at the trailing edges of the vanes as well. Thus, what makes the adhering of the fibers to the leading and trailing edges of the vanes so significant is that the fibers when adhering to the edges result in flocs, threads or strings of several fibers being released from the edge from time to time and being pumped by the pump further in the process. When the process is, for instance, a paper or board making process of pulp and paper industry the flocs, threads or strings enter the web forming stage and remain visible in the end product or they may as well cause a hole in the end product or, as the worst option, a web breakage.
Another problem that was observed when studying impellers used for pumping fibrous suspensions relates to yet other edge areas of the impeller. In other words, it was observed that while the cross section of both working and rear vanes of ordinary centrifugal pumps is, in practice, rectangular, the vanes have at their free ends two relatively sharp edges (applies to semi-open impellers). In a similar manner also the leading and trailing edges of the shroud/s may have sharp edges. Also the center wall of a double-suction impeller normally has sharp edges at its outer circumference. It was learned in the performed experiments that the sharp edges tend to collect fibers. The fibers adhered to the edge/s allow new fibers to adhere, too, either to the sides of the earlier fibers or to the earlier fibers itself. The turbulence caused by the movement of the vanes in the nearhood of the stationary volute/casing creates turbulence that easily starts winding the fibers together whereafter a thread is formed. When such thread/s are released from the edge/s in head box feed pumps of, for instance, a paper or board making process of pulp and paper industry the threads enter the web forming stage and remain visible in the end product or they may as well cause a hole in the end product or, as the worst option, a web breakage.